1. Field of the Invention
This invention relates to a process of recovering indium and, more particularly, to a process of obtaining indium by selectively extracting indium ions from an aqueous solution containing indium ions by a liquid-liquid ion-exchange process and then back-extracting the indium ions into an aqueous solution acidified by sulfuric acid.
2. Description of the Prior Art
Indium does not occur as ores for itself but exists in very small quantities in ores mainly for zinc and lead. Therefore, an industrial raw material for indium is mainly an intermediate product containing concentrated indium by-produced in a smelting step for zinc, etc., from the ores described above. A general process of recovering indium includes a wet process wherein a raw material for indium, i.e., the above-described intermediate by-product is leached by a mineral acid, etc., and an indium component in the leached solution is separated from components of other metals and purified into a concentrated form. As a conventional wet process, there are a neutralization process for the leached solution, an ion-exchange process, and a liquid-liquid ion-exchange process.
The neutralization process is mainly composed of a technique precipitating indium hydroxide from an aqueous leaching solution of the above-described ores but since the product obtained by such a process is poor in purity, it is required for obtaining the commercial grade product to repeat the purification of the indium product composed of many steps such as a crude neutralization, filtration, dissolution, copper removal, neutralization for purification, etc., and hence the process is complicated and lacking in economical value.
The ion-exchange process uses an ion-exchange resin but in the process the separation efficiency between indium and iron is poor and the concentration ratio of indium is low. Furthermore, since the process requires a high concentrated acid solution containing hydrochloric acid or a halide such a chloride for eluting indium from the ion-exchange resin, the process is unsuitable for the treatment of indium-containing solution employed in a general wet-system zinc smeltery. In other words, intermixing of a halide or a halogenic acid in the wet-system zinc smelting system (sulfuric acid system) causes various troubles in the zinc electrolysis step and others.
The liquid-liquid ion-exchange process is a solvent extraction process using an organic solvent immiscible with water and as the organic soluvent used for the process, there are known ether, isobutyl methyl ketone (MIBK), tributylphosporic acid (TBP), a tertiary fatty acid, a monoalkylphosphoric acid, and a dialkylphosphoric acid.
The ether extraction process and MIBK extraction process among the liquid-liquid ion-exchange processes described above cannot be applied to the extraction of indium from a sulfuric acid-acidic solution containing indium but an aqueous halogenic acid solution containing indium is mainly treated by these processes. There are problems in the application of these processes to a general wet-type zinc smelting system. Also, in these processes there is a fault that a large amount of ether or MIBK is dissolved in an aqueous solution treated, which results in increasing the loss of the solvent as well as causing a problem that the solvent dissolved in the aqueous solution gives bad influences on other steps.
The tributylphosphoric acid (TBP) extraction process is mainly employed for the extraction of indium from a hydrochloric acid-acidic solution since the extraction efficiency of indium from a sulfuric acid-acidic solution by the process is very low and the process is relatively widely used in the field without causing the dissolution of the solvent in the aqueous solution. However, the solvents also extract all metals which are able to form chlorocomplex salts and hence in order to recover indium alone, additional positive separation steps are required, which restricts the utilization of the process.
The extraction process using a tertiary fatty acid can be applied to extract indium from aqueous solution other than an aqueous halogenic acid-acidic solution, such as an aqueous sulfuric acid-acidic solution but since the solvent extracts ferric ions (Fe.sup.3+) more preferentially than indium ions, it is necessary to previously reduce ferric ions existing in the aqueous solution into ferrous ions (Fe.sup.2+). Also, the pH at which the solvent extracts indium is limited to a relatively narrow whole range of 2.5-3.5 and the separation efficiency of indium from other heavy metal ions is low, the purity of indium in a back-extracted solution obtained tends to be easily reduced. Consequently, for purifying the back-extracted solution, other process such as the above-described TBP extraction process must be also employed together.
By the conventional extraction process using a monoalkylphosphoric acid and/or a dialkylphosphoric acid, indium ions can be recovered by extraction from an aqueous solution thereof having a relatively high sulfuric acid concentration, i.e., the aqueous sulfuric acid solution (sulfuric acid content of 500-12 g/liter) having a pH of -1.0 to 0.6 pH as shown in FIG. 1 of the accompanying drawings. FIG. 1 is a graph showing the extraction equilibriums of indium ions, ferric ions and zinc ions in the case of mixing 100 ml of each aqueous sulfuric acid-acidic solution having each different sulfuric acid concentration containing 525-588 mg/liter of indium ions (In.sup.3+), 122-225 mg/liter of ferric ions (Fe.sup.3+) and 246-250 mg/liter of zinc ions (Zn.sup.2+) with 100 ml of each organic solvent solution comprising a mixture of di(2-ethylhexyl)phosphoric acid (D2EHPA) and a paraffinic organic solvent, MSB 210 (trade name, made by Shell Chemical Co.) in 5:95 by volume ratio as an extracting solution. In FIG. 1, the axis of ordinate indicates the extractability and the axis abscissa the pH of a sulfuric acid-acidic solution.
As is understood from the figure, under the pH condition for extracting indium ions in the above-described process, ferric ions are simultaneously extracted and hence it must be considered to previously reduce the ferric ions dissolved in an aqueous solution to ferrous ions. However, it is not always easily to completely reduce the ferric ions in an aqueous solution, the accumulation of ferric ions in the solvent repeatedly used in practical step is unavoidable, and hence the employment of back-extraction or stripping must be considered. For the back-extraction of ferric ions from an extracted solvent solution, an aqueous sulfuric acid-acidic solution cannot be substantially used and hence a halogenic solution, intermixing of which in an indium extraction system is undesirable, must be employed at present in a wet-system zinc smeltery as described above. Furthermore, the back-extraction of indium ion itself requires two kinds of halogenic acid solutions each having different concentration and thus, two stages of back-extraction steps for ferric ions and indium ions are ultimately required.